Ancient people had items which were not for any practical use apart from adornment and decoration. So if we imagine Ug has hunted enough animals to have meat, skins, etc. for his own needs, but sees that Thag is shivering, and in need of a nice warm fur and also has a nice set of cowrie shells around his neck...they get together, and make the exchange. Later on, W'gaag sees Ug's set of shells, and offers him some of the fish she's caught for them. Ug fancies a change of diet and is bored with the shells. Bingo, the root of all evil is born...